1. Field of the Invention
The present invention relates to a method for the exchange in real time of digital signals between two digital systems having sampling timings which are asynchronous and have differing cycles.
2. Background Art
In communications using digital circuits represented by ISDN, the sampling of an analog signal and the clock synchronizations of data at the terminal side are conducted based on a timing signal supplied from the network side. This presents no problems with respect to a terminal having a system which was designed from the outset to use digital circuits; however, in the case in which a terminal using analog circuits is connected to digital circuits, this connection is usually accomplished through the medium of an analog signal. For example, in a GIII facsimile, which is defined by the CCITT recommendation T.4, the terminal in the inner digital signal processing part is connected to analog circuit terminals, to convert the digital signal to analog signals by a modem (see FIG. 1).
In the above technique, in the case in which the sampling cycle of a digital signal is changed, a method is used in which the digital signal which is to be processed is converted to an analog signal by a D/A (digital/analog) converter, the high frequency signal contained in the analog signal is eliminated by an analog filter, and the analog signal obtained as a result undergoes sampling at a desired sampling cycle, and then is converted to a digital signal by an A/D (analog/digital) converter.
As in GIII facsimiles defined by the CCITT recommendation T.4, in the case in which a terminal in the interiors of which digital signal processing is conducted is connected to digital circuits, the connection thereof by digital signals has been considered.
However, at the time of the realization thereof, there is a problem in that the terminal and the network use mutually different sampling timings. The modem used in GIII facsimiles operates according to an individual sampling timing applicable to its signal processing and the cycle thereof is normally different from that of the timing signal supplied from the network.
Furthermore, in order that the modem of the GIII facsimile receive a simultaneous signal, it is necessary that the sampling timing be so set as to permit the control thereof by the modem.
In addition, the conventional conversion method for sampling cycles by an analog signal utilizes analog circuits such as D/A converters, analog filters, and A/D converters, and the like, so that the quality of the digital signal after conversion is affected by the variance in performance of these analog circuits. Furthermore, in order to use analog circuits in a digital system, it is necessary to take countermeasures so that the noise of the digital system will not enter the analog circuits, and for this reason ,a very high degree of mounting technology is required.